1. Field of the Invention
This invention relates to aircraft terrain and obstacle avoidance systems.
2. Description of Related Art
Each year there occur aircraft crashes in unacceptable numbers and this number has on average shown no significant sign of diminishing. In the first six months of 1990 there were eighteen such crash incidents involving civil transport passenger carrying aircraft alone (Flight 25th July, 1990). Investigations into the causes of aircraft crash incidents reveal that in many cases the aircraft is operating normally, the cause of the crash incident not being attributable to a system fault. In these types of incidents, graphically often referred to as Controlled Flights into Terrain, the cause is given as pilot error.
And, although the pilot may have contributed to the event, had he been given sufficient warning that, for whatever reason, the aircraft was in imminent danger of crashing, evasive action could have been taken and ground contact avoided.
Large civil passenger carrying aircraft are already fitted with Ground Collision Avoidance Systems (GCAS). These systems are regarded by aircrew members often as unsatisfactory in that they give rise, not infrequently, to nuisance warnings causing the pilot to take spurious evasive action in avoiding a non-existent ground collision risk.
As a result of such experiences warnings generated may tend not infrequently to be ignored by aircrew or the system may even be disabled.
Clearly, such a state of affairs is unacceptable, and a pressing need arises for an aircraft terrain and obstacle avoidance system which is effective reliable and the alarm output of which is believable and acted on by the pilot with a low probability that the warning is a spurious one.
From the foregoing it will be appreciated that desiderata for such a system are: to provide the pilot with a timely warning of impending collision risk, timely in the sense of allowing sufficient opportunity for evasive action to be taken; it must exhibit a low false alarm rate if it is to be credible and instill confidence in the user; it must operate whatever the aircraft altitude or the flight path maybe; in military applications, the system must continue to operate even if the aircraft is in a steep dive or is flying inverted and so that the pilot is able to maintain complete control of the aircraft so that he may take advantage of the full manoeuvrability of the aircraft in executing evasive action in an hostile environment; and, for production purposes, the system should be flexible so as to find application on different types of aircraft possessing different flight characteristics and capabilities. A further desirable characteristic of particular value in military applications is that the system should be adjustable by the pilot to accommodate variable severity in vertical acceleration in a pull-up manoeuvre within stress limits which might be imposed on the airframe.